The present invention generally relates to a magnetic recording and reproducing apparatus such as a video tape recorder or the like and more particularly, to a reel base driving mechanism for use in the magnetic recording and reproducing apparatus.
A reel base driving mechanism for use in a magnetic recording and reproducing apparatus is known from, for example, Japanese Utility Model Publication No. 31290/1973 (Jikkosho 48-31290) in which an idler is pivotally provided. One example of application of the known reel base driving mechanism to a video tape recorder is shown in FIG. 1 in which a tape cassette 1, a guide cylinder 3, a supply reel base 4 for rotating a tape supply reel, a takeup reel base 5 for rotating a tape takeup reel, an idler 6 and an idler lever 7 for supporting the idler 6 are provided such that a video tape 2 drawn out of the cassette 1 is wound over a predetermined angle around the guide cylinder 3. The idler 6 is selectively and pivotally brought into rolling contact, through rotation of a motor pulley 8 in pressing contact with the idler 6, with the supply reel base 4 and the takeup reel base 5 so as to be pivoted together with the idler lever 7 about the motor pulley 8. Meanwhile, a friction member (not shown) made of felt, plastic magnet or the like is provided between the idler 6 and the idler lever 7.
On the other hand, in response to recent diversified development of the video tape recorders, there have been proposed vertical setting type video tape recorders such as portable video tape recorders, vertical type video tape recorders, etc., each provided with a mechanism chassis having a component mounting face oriented vertically, in addition to horizontal setting type video tape recorders such as conventional fixed type video tape recorders, each provided with a mechanism chassis having a component mounting face oriented horizontally. In the case where the reel base driving mechanism of FIG. 1 is employed in the vertical setting type video tape recorders, it is usually so arranged that the cassette 1 is loaded into the video tape recorders such that a tape outlet side of the cassette 1 is oriented downwardly. As shown in FIG. 2, supposing that centers of gravity of the idler 6 and the idler lever 7 are P1 and P2, respectively, gravitational forces applied to the centers P1 and P2 of gravity are F1 and F2, respectively, lengths between a pivotal center O of the idler lever 7 and the center P1 of gravity and between the pivotal center O and the center P2 of gravity are L1 and L2, respectively and an angle formed by an ordinate Y-Y' and a straight line connecting, via the pivotal center O, the centers P1 and P2 of gravity is .theta., a turning moment M1 of the idler 6 and a turning moment M2 of the idler lever 7 are given by: EQU M1=F1 L1 sin .theta. EQU M2=F2 L2 sin .theta.
When the idler 6 is simply placed on the idler lever 7, the gravitational force F1 is apt to be larger than the gravitational force F2 and thus, the turning moment M1 becomes larger than the turning moment M2. Namely, since a center P of gravity of combination of the idler lever 7 and the idler 6 is disposed on a line O-Pl below an abscissa X-X', a clockwise turning moment is generated so as to reduce a pressing contact force applied from the idler 6 to the supply reel base 4. Since this pressing contact force is obtained by a frictional force of the friction member disposed between the idler lever 7 and the idler 6, the frictional force of the friction member is required to be increased in order to prevent reduction of the above described pressing contact force. Thus, the known vertical setting type video tape recorders have such inconveniences that since a load of a drive motor, i.e., a reel motor increases when the frictional force of the friction member is enlarged, power consumption of the reel motor increases and the reel motor cannot be made compact in size.